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The concrete base of the 541 metre Freedom Tower, being built on the site of New York's former World Trade Centre, will be embedded with RFID tags to make sure it's setting properly and can sustain the pressure of 14,000 psi the tower will exert.

Monitoring the temperature of setting concrete is nothing new, but doing so wirelessly is a developing technique that was first trialled in 2003. The technology being used for the Freedom Tower comes from Wake Inc, which leads the relatively new field.

The wireless solution is more expensive, but the complexity of laying cables and keeping track of them largely offsets that expense.

The tags themselves are battery powered, which gives a much greater range (up to 300 feet according to the manufacturers) than the more familiar induction-powered tags, but limits the life to about six years. Where tags are powered by induction, the tag reader needs to be within a few inches, something hard to achieve when the parts of the structure are 20 feet thick.

The Freedom Tower is due to open in 2011, so the tags should still be functioning come opening day, which might give comfort to the first people to ascend. ®